1. Field of the Disclosure
The disclosure is related generally to the use of resistivity measurements for identification of fracturing and determination of the extent of fracturing in earth formations and establishing true formation resistivity without fracturing.
2. Background of the Art
In exploration for hydrocarbons, a significant number of reservoirs involve fractured reservoirs. Broadly speaking, there are two types of situations encountered in development of such reservoirs. The first case involves a rock matrix that has a significant porosity so that the hydrocarbons occur within the pore spaces of the rock matrix; however, the permeability of the matrix itself is very low, making development of such reservoirs uneconomical. In such rocks, permeability resulting from fracturing of the rock matrix may make commercial development economical. A second case involves reservoirs in which the only significant porosity in the reservoir is due to fracturing of the rock matrix. Examples of reservoirs that produce from fractured granite are the Playa Del Rey field and the Wilmington field in California, and the Hugoton field in Kansas. It is thus important to be able to identify the extent of fracturing in earth formations.
Fractures observed in boreholes hold important clues for the development of a field. Open natural fractures may enhance productivity in the case of depletion drive or lead to early water breakthrough under a water drive or strong aquifer scenario. However, cemented fractures may form barriers to flow. Therefore it is important to know the length of natural fractures to allow for optimized field development. Drilling induced fractures can also be observed in a wellbore. This information can be used to determine the direction in which hydraulic fractures employed in the development of tight reservoirs will propagate. The actual hydraulic fractures can be monitored with micro-seismic, which is relatively expensive and requires a monitoring well close by.
Drilling induced fractures are frequently generated by heavy mud and/or drilling force. Characterization of borehole fractures is important since they reflect the formation stresses. The appearance of fractures filled with conductive or resistive fluid changes the original formation resistivity distribution around the wellbore. This resistivity change affects the multiple depths of investigation measurements of the induction tools differently depending on fractures conductivity, inclination, orientation, width, length, and density. To accurately characterize undisturbed formation resistivity, corrections for the fracture presence is required.
There have been numerous attempts at characterizing fractures and determining formation resistivity. U.S. Pat. No. 5,574,218 to Withers discloses the use of seismic methods to determine the azimuth and length of a hydraulic fracture. No determination of formation resistivity is done. There are many references directed at determining horizontal and vertical resistivities of anisotropic formations. See, for example, U.S. Pat. No. 6,502,036 to Zhang et al., having the same assignee as the present disclosure, and U.S. Pat. No. 6,643,589 to Zhang et al., having the same assignee as the present disclosure,. U.S. Pat. No. 6,925,031 to Kriegshauser et al., having the same assignee as the present disclosure, uses acoustic and multicomponent 3DEX® to determine fracture properties. However, the prior art does not address the problem of simultaneously determining fracture properties and resistivities of the formation.
The present disclosure deals with the need for determination of fracture properties and true formation resistivities using induction data.